Many proposals have been made regarding hollow fibers having holes which communicate from the surface of the fibers to hollow portions thereof. For instance, a water absorptive fiber is disclosed in Japanese Examined Patent Publication No. 61-60188 in which polyester hollow fibers blended with an organic sulfonic acid metal salt are subjected to an alkali treatment to dissolve off the organic sulfonic acid metal salt and to form, as traces of the removed salt, micropores having a diameter of 5 .mu.m and communicating to hollow portions.
However, there were problems that since the communicating pores obtained by this method are extremely fine, they scarcely affect the hand feeling of the hollow fibers and that there is a limit in the improvement of water absorptive property. Further, since the microfine pores are almost uniformly formed across the entire cross-section of the fibers according to this method, there was a problem that the fibers are liable to become fibrils which deteriorates their physical properties.
In order to solve these problems, a hollow fiber has been proposed in which through grooves (microgrooves) or cracks (slits) are formed from the fiber surface to hollow portions thereof. For instance, it has been disclosed in Japanese Unexamined Patent Publication No. 56-169817 that a fiber having an excellent water absorptive property is obtained by treating a sheath-core type composite fiber wherein a nylon covered with a polyester is treated with a solvent for nylon to form cracks which pass through from the fiber surface to hollow portions therein and are formed parallel to the fiber axis. Further, it has been disclosed in Japanese Examined Patent Publication NO. 60-37203 that a water absorptive fiber is obtained by applying a twisting force to composite fibers having the structure mentioned above, and then dissolving off a part of the core portion. Still further, it has been disclosed in Japanese Unexamined Patent Publication No. 5-44160 that a part of the core component in the composite fiber mentioned above is exposed to make the dissolution of the core component easy.
Incidentally, in all of the proposals mentioned above, since sheath-core type fibers in which the polymer in the sheath portion has a weight reduction rate with an alkali different from that of the core portion, such extremely complicated steps in spinning technology, called composite spinning, must be used. In addition, since the difficulty inevitably arises in these methods that the polymer in the core portion cannot completely be removed and that the removal ratio of the polymer in the core portion is dispersed, there have been problems that uneven dyeing occurs and that deterioration of the physical properties and abrasion resistance of the hollow fibers themselves occur, and thus the fibers may not withstand practical use.